1. Field of the Invention
The present invention relates to a breathing apparatus suitable for full-face masks and half-face masks used with the object of protecting from dust, gases, and the like.
2. Description of the Related Art
People working in the atmosphere containing hazardous dust or toxic gases usually wear a dust mask or a gas mask and inhale a purified air after the hazardous and toxic substances contained in the air have been removed-with a filtration material such as active carbon or a filter contained in the dust mask.
However, filtration materials such as filters, absorption canisters, and the like, with good purification efficiency typically have a large draft resistance.
In particular, because penetration of radioactive dust present in nuclear power plants, dioxin-containing toxic dust in decomposition sites of incinerators, and toxic gases generated in a variety of other industrial operations into a human body affects human health, filtration materials with a high purification efficiency and, therefore, a high draft resistance are used for dust masks. People wearing dust masks provided with such filtration materials have difficulty in breathing normally by using only the capacity of their lungs.
Accordingly, a blower operated by electric power has been mounted on a dust mask in front or behind the filtration material in a draft channel and the suction force created by the rotation of the blower facilitated breathing.
However, the following problems are associated with such conventional technology.
(1) Toxic substances penetrate into the human body via trachea essentially only during inhalation. Therefore, the filtration material may operate only during inhalation.
In the dust masks comprising no blower, because the exhaled air is let out by an exhalation valve, the filtration material is not exhausted during exhalation. On the other hand, since in the dust masks equipped with a blower, the blower operates also during exhalation, the filtration material is exhausted faster than in the dust mask comprising no blower.
(2) Human breathing requires 0.45 to 0.68 L of air for a single inhalation of an adult person. The frequency of inhalation is typically about 12 to 16 per minute. In particular, masks are used most often during work and the breathing volume increases in proportion to the volume of work. The maximum draft volume during inhalation can be higher than 85 L/min at the peak.
However, if the voltage supplied to the blower is set such that the ventilation amount of the blower is no less than the maximum draft volume, the electric power consumed by the blower unnecessarily increases and the exhaustion of the filtration material is accelerated. Further, because the filtration materials with a high draft resistance require blowers with a high torque, the consumption of electric power increases in proportion to the draft resistance of the filtration material used.
(3) In the conventional dust masks equipped with a blower, the air is supplied into the dust mask also during exhalation. As a result, a positive pressure is created in the facepiece of the dust mask. In particular, if the ventilation volume of the blower is set higher than the maximum peak of breathing, the pressure in the facepiece becomes very high and the exhalation resistance is increased.
On the other hand, in the conventional dust masks comprising no blower, the exhalation resistance is practically equal to the exhalation valve resistance, and the exhalation resistance is typically lower than that in the above-described dust masks equipped with a blower.
A breathing apparatus (mask for breathing) comprising a fan driven by a motor, a filter arranged opposite the fan, and a mask facepiece receiving the air that passed through the filter has been disclosed in Japanese Patent Application Laid-open No. H2-74267 (and in the U.S. Pat. No. 4,971,053 corresponding thereto). This breathing apparatus also comprises a differential pressure sensor composed of a pressure-responsive member (diaphragm) connected so that one side thereof faces the pressure downstream of the fan and the other side faces the pressure upstream of the fan, and control means for controlling the operation of the fan motor in response to the differential pressure sensor.
However, in such a breathing apparatus, the first channel connecting one side of the pressure-responsive member to the zone downstream of the fan and the second channel connecting the other side of the pressure-responsive member to the zone upstream of the fan have to be provided separately from the main inhalation channel. As a result, the mask structure is very complex and the differential pressure sensor is difficult to mount in a compact manner. Further, because the opening of the first channel or second channel had to be provided between the filter and the fan, the size of the entire breathing apparatus was inevitably increased. Further, a diaphragm is used as the pressure-responsive member, but the diaphragm is easily fatigued or damaged and the set values of the reaction pressure of the differential pressure sensor are difficult to maintain.